Dihydroxybenzenes are used in the manufacture of dyes in the plastics industry, photographic industry and agricultural pesticide industry.
German Patent Specification No. 20 64 497 discloses a method of nuclear hydroxylation of aromatic compounds, especially of phenol, with hydrogen peroxide in the presence of a strong acid. In this method, the reaction medium should not contain more that 20% by weight water initially, preferably less than 10% by weight water.
The pH of the strong acids is indicated to be under -0.1, preferably under -1. Sulfuric acid and perchloric acid appear to be preferred strong acids.
However, this method is criticized in by the applicant of German Patent Specification DE-PS No. 20 64 497 himself in his later German Patent Specification DE-PS No. 26 58 545. Thus, the yields of hydroxylation products are said to be "excellent" with the simultaneous usage of the strong acids and of complexing agents for metals like pyrophosphoric acid; however, the degree of conversion of the aromatic compound is under 30%. In practice, conversions of 4 to 10% were not exceeded.
This results in a limitation on the productivity of the apparatus and the recovery of a significant volume of unused raw material.
As high a reaction speed as possible would be desirable. This would depend, at a given temperature and amount of water, on the type and amount of the acid used. However, it would be desirable, regardless of the type of acid, to increase the reaction speed without increasing the amount of acid, since the latter would be lost by being washed out and, moreover, the corrosion due to the strong acid is undesirable.
Thus, DE-PS No. 26 58 545 suggests, as an improvement of the above-mentioned method, in addition to the catalysts and stabilizers cited in it, the addition of aromatic aldehydes such as benzaldehyde to the reaction mixture. The hydroxylation of phenol and substituted phenols with hydrogen peroxide thus occurs in the presence of strong acids, metal complexing agents and aromatic aldehydes.
Instead of two components which influence the reaction, three must therefore be used. These components must not only be separated out of the reaction mixture and are lost as non-recoverable but, in addition, the "aromatic aldehyde" component is subject to an oxidation with hydrogen peroxide, which leads to the risk of contamination of the final product.
However, higher yields of approximately 70-76% and in two instances over 80% are also only obtained here if phenol/hydrogen peroxide ratios of 20:1 and hydrogen peroxide of approximately 85% concentration are used. However, in these instances "a significant volume of initial material must be recovered", as in DE-PS No. 20 64 497, which necessitates recovery systems. Even the reactors themselves must be designed to be correspondingly large.
Since the yield of catechol and hydroquinone drops considerably in the method of DE-PS No. 20 64 497 as the ratio of phenol to hydrogen peroxide decreases--at a ratio of 10:1 it is 60% and at a ratio of 5:1 at 47%, cf. example 7--the presence of a large excess of phenol appears to be a necessity when performing the hydroxylation with strong acid, quite apart from the fact that the strong acids such as sulfuric acid and perchloric acid named in DE-PS No. 20 64 497 only had a weak catalytic action. The yields were even worse in the case of substituted phenols.
The assumption was therefore made that the catalytic action of the strong acids could be improved by using water-free solutions of hydrogen peroxide--cf. German Patent Specifications DE-PS No. 24 10 742 and DE-OS No. 24 10 758; however, the presence of phosphorus compounds as complexing agents was also considered to be essential here. In addition, it was pointed out that the reaction occurred the fastest at high concentrations of acid. The question of corrosion by strong acids was therefore also not solved here.
A considerable advance over the mentioned methods is represented by the methods of German Patent Specifications DE-PS No. 33 08 737 (corresponding to U.S. Pat. No. 4,618,730); DE-PS No. 33 08 769 (corresponding to U.S. Pat. No. 4,533,766); DE-PS No. 33 08 763 (corresponding to U.S. Pat. No. 4,590,305) and DE-PS No. 33 08 726 (corresponding to U.S. Pat. No. 4,551,562) in which the hydroxylation of phenol or its derviatives was performed by organic solutions of hydrogen peroxide in the presence of sulfur dioxide or selenium dioxide.
The undesirable consequences of using strong acids, such as corrosion, did not occur in these methods, nor was it necessary to use additional compounds such as aldehydes or even complexing agents such as phosphorus derivatives in order to increase the activity.
Although the catalysts were used in very small amounts, the reaction speed was high; very advantageous space-time yields and very good yields were obtained. Due to their very small concentration, sulfur dioxide and selenium dioxide also require no special separation methods.
According to the state of the art, therefore, only the usage of water-free solutions of hydrogen peroxide appeared at first to result in better conversions and yields. However, when strong acids were used as catalyst, there was still the problem of separating them out as well as the occurrence of corrosion.
The usage of sulfur dioxide or selenium dioxide made a considerable advance here. However, even these methods required water-free solutions of hydrogen peroxide which were not supposed to contain more than 1% by weight, preferably less than 0.5%, by weight water. This method of operation also requires additional systems for separating and recycling the organic solvent.